In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. In addition to drill pipe sections, the drill string for non-horizontal drilling operations typically incorporates heavier tubular members below the pipe sections known as heavyweight pipe or drill collars, disposed near the end of the drill string above the drill bit to increase the vertical load on the drill bit and thus enhance the bit's operational effectiveness. Other accessories commonly incorporated into drill strings include stabilizers to assist in maintaining the desired direction of the drilled borehole, and reamers to ensure that the drilled borehole is maintained at a desired gauge (i.e., diameter). In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the borehole, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall).
Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottom hole assemblies” are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill siring immediately above the drill bit. A typical downhole motor includes several primary components, as follows (in order, starting from the top of the motor assembly):                a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);        a power section;        a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and        a bearing assembly (which includes a mandrel with an upper end coupled to the lower end of the drive shaft, plus a lower end adapted to receive a drill bit).        
In drilling processes using a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. As will be described in greater detail below, however, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the downhole motor to generate power to rotate the drill bit.
The main components of the power section are an elongate and generally cylindrical stator disposed within a tubular stator housing, and an elongate rotor member rotatable within the stator. The power section in the most common type of downhole motor is a “positive displacement” power section, which is essentially similar to the progressive cavity (or “Moineau”) pump well known in the art, but operating in reverse. The rotor comprises a shaft formed with one or more helical vanes or lobes encircling the shaft and extending along its length. The stator is typically in the form of an elastomer lining bonded to the inner cylindrical wall of the stator housing, and defines helical lobes of a configuration complementary to that of the rotor lobe or lobes, but numbering one more than the number of rotor lobes. The lower end of the rotor comprises or is connected to an output shaft which is in turn connected to the upper end of the drive shaft.
During operation of the downhole motor, high-pressure drilling fluid is forced through the power section, causing the rotor to rotate within the stator, and inducing a pressure drop across the power section (i.e., the drilling fluid pressure being lower at the bottom of the power section). The power thus delivered to the rotor output shaft is proportional to the product of the volume of fluid passing through the power section multiplied by (the pressure drop across the power section (i.e., from fluid inlet to fluid outlet). Accordingly, a higher rate of fluid circulation fluid through the power section will result in a higher rotational speed of the rotor within the stator, and correspondingly higher power output.
As previously noted, the output shaft of the power section rotor is coupled to the upper end of the drive shaft, for transmission of rotational torque to turn the drill bit. However, the motion of the rotor in a positive displacement-type downhole motor (or “PD motor”) is eccentric in nature, or “precessional”—i.e., in operation, the lower end of the rotor (i.e., the output end) rotates about the centroidal longitudinal axis of the stator housing, such that the longitudinal axis of the rotor rotates in an eccentric motion about the stator axis, defining a conical surface of rotation.
In an alternative type of power section for a downhole motor, the rotor and stator of a power section incorporate turbine blading, with the stator comprising stationary blade sections fixed within and to the tubular stator housing, and with rotor blade sections fixed onto a rotor shaft. Drilling fluid is forced through the stator under pressure, inducing rotation of the rotor within the stator. In this turbine-type power section, the rotor axis remains coincident with the axis of the stator, so there is no eccentric or precessional motion as in the case of a PD motor power section, and the rotational speed is much higher. Although a turbine-type power section does not have the same characteristics as a PD motor power section, it performs the same basic function (i.e., to produce rotational power to turn the drill bit) by essentially similar means (i.e., rotor rotation induced by passage of pressurized drilling fluid through the stator).
Irrespective of the particular type of power section used in the downhole motor, the lower or output shaft of the rotor is operationally coupled to the upper end of the drive shaft by means of a first (or upper) universal joint, whereby rotational torque can be transferred from the rotor to the drive shaft irrespective of the fact that the rotor and drive shaft axes may be non-coincident as necessary for purposes of directional drilling (as well as due to the rotor's eccentric rotation in the case of a PD motor power section). When a turbine power section is employed, and the drive shaft housing is a solid, one-piece housing with no bend, a universal joint is not required.
The bearing assembly typically incorporates an elongate tubular mandrel having an upper end which is operationally coupled to the lower end of the drive shaft by means of a second (or lower) universal joint, and a lower end to which a drill bit may be mounted. The mandrel is encased in a tubular bearing housing that connects to the tubular drive shall housing above. The mandrel rotates non-eccentrically within the bearing housing. As the other components and construction details of the typical bearing assembly are not pertinent to the present invention, they will not be discussed further herein; however, they will be well known and understood by persons of ordinary skill in the art of the invention.
In operation, the drive shaft and power section rotor in a downhole motor are subjected to high axial compression loads as the drill bit bores into subsurface formations to create a borehole. The magnitude of these compressive loads will vary with the power generated by the downhole motor, which as previously discussed is proportional to the pressure drop across the power section. To accommodate these compressive loads, a thrust bearing of some type is typically incorporated into the universal joint assemblies at each end of the drive shaft.
The transverse dimensions of a universal joint for a downhole motor drive shaft are inherently constrained by the internal diameter of the drive shaft housing. Therefore, a drive shaft universal joint must incorporate sufficient structure for effective transfer of in-service torque and thrust loads, while being sufficiently compact in size to prevent physical interference with the drive shaft housing due to “wobbling” or precessional movement of the output end of the power section rotor. Accordingly, the inner diameter of (the drive shaft housing constitutes a fixed design constraint when it is desired to increase the torque and/or thrust capacity of a drive shaft universal joint, or to increase the fatigue strength or service life of a universal joint for given torque and thrust loads.
The prior art discloses numerous examples of universal joints for use in association with downhole motor drive shafts. For example, Canadian Patent No. 1,290,952 (Wenzel) and corresponding U.S. Pat. No. 4,772,246 disclose a drive shaft universal joint that uses four articulating drive keys having cylindrical shanks swivellably disposed within corresponding radial sockets in the side of the drive shaft. Each drive key has a head portion slidingly disposed within a longitudinal slot in an end housing surrounding the joint, with a pair of opposed planar side surfaces in sliding contact with planar sidewalls of the longitudinal slot, for transferring torque to the end housing. The heads of the drive keys will slide within their corresponding slots in the housing in response to any angular deviation between the axes of the drive shaft and the end housing, with all four drive keys remaining-in contact with the slot sidewalls for transfer of torque. A spherical ball bearing serving as a thrust bearing is placed on a bearing scat centrally located within the central shaft, and with the axes of the drive key shanks passing through the center of the thrust bearing.
Canadian Patent No. 2,023,042 (Wenzel et al.), and corresponding U.S. Pat. No. 5,267,905 disclose a drive shaft universal joint that transfers torque by means of ball bearings each disposed partially within a hemispherical pocket in the side of the drive shaft and partially within a semi-cylindrical longitudinal slot in an end housing surrounding the joint. The ball bearings are equally spaced in a circular pattern around the drive shaft, lying in a plane perpendicularly transverse to the axis of the drive shaft. The universal joint further includes a ball thrust bearing centrally positioned within the drive shall, with the center of the thrust bearing aligned with the centers of the ball bearings.
In the universal joint of CA 2,032,042, the only time all of the ball bearings are effectively engaged for transferring torque is when the longitudinal axes of the end housing and the drive shaft are coincident, a condition that never exists in a PD motor due to the precessional motion of the power section output shaft as previously discussed. Whenever there is a relative angular deviation between the axes of the power section rotor and the drive shaft, there will be at most two (diametrically-opposed) torque-transmitting balls fully engaged for effective torque transfer. This is because the angular deviation causes the balls to define an ellipse rather than a circle when viewed along the axis of the end housing. Accordingly, some of the balls will be displaced radially inward, out of their slots in the end housing, in response to any angular deviation between the rotor and drive shaft axes, such that only those balls disposed at or near the larger diameter of the ellipse will be in fully effective engagement with their corresponding slots in the end housing. In addition to the resultant inefficiency of torque transfer, the lack of constant engaging contact between the drive balls and the housing increases shock loading on the assembly, inducing fatigue stresses lending to shorten the service life of the universal joint.
U.S. Pat. No. 5,704,838 (Teale) teaches a further example of a drive shaft universal joint that uses ball bearings as a torque transfer means. As in CA 2,023,042, however, the Teale U-joint it has the drawback that only two torque-transmitting balls will be fully effective for torque transfer at any given time whenever the axes of the connecting shafts are not coincident.
Additional examples of prior art universal joints may be found in the following:                Canadian Patent Application No. 2,541,339 (Johnson et al.);        U.S. Pat. No. 4.904,228 (Frear et al.);        U.S. Pat. No. 4,982,801 (Zitka et al.);        U.S. Pat. No. 5,000,723 (Livingstone);        U.S. Pat. No. 5,048,622 (Ide);        U.S. Pat. No. 5,078,650 (Foote);        U.S. Pat. No. 5,288,271 (Nelson et al.); and        U.S. Pat. No. 7,186,182 (Wenzel et al.).        
The prior art universal joints fail to address several significant problems, the first of which is non-uniform and inefficient torque transfer in U-joints using circularly-arrayed torque-transfer elements such as the torque-transfer balls in CA 2,032,042 and U.S. Pat. No. 5,704,838, as discussed above. This problem does not occur in the U-joint of CA 1,290,952 (and U.S. Pat. No. 4,772,246), which incorporates drive keys that remain in sliding-fit contact with corresponding end housing slots in response to angular deviations between the drive shaft and the end housing axes. However, all of the prior art U-joints, including the U-joint of CA 1,290,952, are limited in their torque-transfer capacity by both the available shear strength of their torque-transfer elements (e.g., drive balls or drive keys) and the effective contact area available for transferring torque-induced forces between the torque transfer elements and their corresponding resistive elements (e.g., pockets or slots in drive shafts or end housings). Such torque-transfer limitations severely restrict the suitability and serviceability of known U-joints for use with higher-power PD motors that are now available in the market.
A further problem arises with known U-joints of the type that uses a spherical thrust bearing centrally disposed between the drive shaft and the end housing, as in CA 1,290,952, CA 2,032,042, CA 2,541,339, and U.S. Pat. No. 5,704,838. Modern PD motor power sections are capable of producing higher power—and therefore higher torque and higher thrust loads—without increasing the overall motor diameter. Known U-joint designs cannot be readily adapted to withstand the higher thrust loads associated with newer high-power, high-torque PD power sections because the size of the spherical thrust bearing is constrained by the diameter of permissible drive shaft U-joints.
For the foregoing reasons, there is a need for an improved PD motor drive shaft and universal joint assembly that can withstand both higher torque loads and higher axial thrust loads than known universal joint assemblies, without necessitating an increase in the diameter of the overall universal joint assembly, and while ensuring uniform transfer of torque forces irrespective of angular deviations between the axes of the drive shaft and the drive shaft end housing. The present invention is directed to these needs.